Apparatus embodying electric discharge lamps



May 2, 1933.

c. o. BROWNE APPARATUS EMBODYING ELECTRIC DISCHARGE LAMPS Filed Oct. 23,1929 Patented May. 2, 1933' UNITED STATES PATENT OFFICE CECIL OSWALDBaowNE, or EALING common, noimon, ENGL ND, AssIeNoR. IBY MESNEASSIGNMENTS, 'ro RADIQCORPORATION or AMERICA, or NEW YORK, N. Y.,

A CORPORATION OF. DELAWARE APPARATUS, monrme ELECTRIC DISCHARGE LAMPSApplication filed October 23, 1929, Serial No. 401,931, and in GreatBritain November 26, 1928.

The present invention relates to apparatus embodying electric dischargelamps, such as glow or are discharge lamps, in which the intensity ofthe light emitted by'the lamp is y varied in accordance with electricoscillations.

Where a mercury vapour discharge lamp is used as a lightsource inphotographic sound recording, a steady voltage is impressed on the lampin order to light it and electrical oscillations corresponding to thesounds to be recorded are superposed upon the steady voltage and serveto vary the light emitted in aceordance with the sound.

It will be clear that the superposed oscillations will produceoscillatory changes in the steady voltage and cause it to vary betweenmaximum and minimum values which are equally above and below the steadyvoltage respectively. Since there is a critical voltage (known as thecut-off voltage) below which the lamp does not remain alight it will beclear that if the minimum value of the voltage is, with normal sounds,close to the critical voltage, an abnormally loud sound will causeextinction of the lamp with consequent distortion of the recorded soundwaves.

According to the present invention there is provided apparatuscomprising an electric discharge lamp the steady illumination of 0 whichis adapted to be varied in accordance with electrical oscillations orvariations, wherein there is associated with the oscillatory or variablecurrent circuit a current limiting device adapted to prevent or tominimize the risk of the voltage across said lamp falling below thecut-off value upon increase in the amplitude of said electricaloscillations or variations. i

The invention is illustrated in the accompanying drawing in which Figs.1 and 3 are diagrams of two difierent circuits in accordance with theinvention, and Figs. 2 and 4 are diagrams illustrating the working ofthe circuits in Figs. 1 and 3 respectively.

Referring to Fig. 1, electrical oscillations, for example from amicrophone, are applied to the terminals 2 and are thus impressed uponthe grid circuit of the valve 3. This valve 59 may be the last valve ofa multi-stage amplifier. The anode circuit of the valve 3 con-- tainsthe primary winding of a transformer 4. The secondary winding of thetrans former 4 is connected to a discharge tube 5, a stabilizingresistance 6 and a battery 7 arranged in series. The battery 7 providesthe current for the steady illumination-of the discharge tube 5. Gridbias for the valve 3 is supplied through a grid leak 8 and is furnishedby battery 9 shunted by a potential divider 10. The resistance 10 andthe battery 9 are shunted by a by-pass condenser 11.

The grid bias upon the valve3 is adjusted by means of the potentialdivider 10 so that the valve3 tends to overload or rectify on thenegative half swing when the voltage swing upon the grid circuit of thevalve exceeds a value which is suflicient to reduce the potentialdiflerence across the discharge tube 5 to the cut-off value or slightlyabove this. This isillustrated in Fig. 2 in which, in curve 12, theanode current i, of the valve 3 is plotted against the grid voltage V,;.The anode current will, for the purpose of this specification beregarded as proportional to the potential difference across the primarywinding of thetransformer 4. The anode current corresponding to theminimum value of grid voltage V is that which represents a potentialdifference across the discharge tube 5 in opposition to that produced bythe battery 7 or the operating potential for the lamp and having amagnitude less than that produced by the battery 7 by an amount equal toor rather greater than the cut-01f voltage of the tube 5.

The grid bias is therefore adjusted to the value v and the grid swing isthen represented by the curve 13. The value of the potential differencesapplied to theterminals 2 is such that normally it does not exceed thevalue given by the .curve 13. That is to say, when the potentialdifferences correspond to sounds, 4 the potential differences shown incurve 13 correspond to the loudest sounds with which it is expected tohave to deal. If, through any unforeseen circumstances, the volumeexceeds this value, it will be seen from curve 12 that the valve 3willact as a rectifying 100 or overloaded current limiter in the regionof V, to prevent extinction of the tube 5.

Referring to Fig. 3, the modulating oscillations are fed to terminals 2which are connected to the primary winding of a transformer 14. Thesecondary winding of the transformer 14 is connected to a resistance 15,a diode rectifier 16, a discharge tube 5 and a battery 17 all arrangedin series. The steady illumination of the tube 5 is provided by battery7 through a stabilizing resistance 6 and a choke 18 which prevents thecirculation of oscillatory currents in the circuit 7, 6, 18.

The voltage of battery 17 is so adjusted that the diode 16 commences torectify when the electromotive forces in the modulating circuit exceed acertain predetermined value which causes the potential difference acrossthe tube 5 to fall, on one half swing, to a value equal to or slightlygreater than the cut-off voltage.

This is illustrated in Fig. 4 in which the curve 19 shows the relationbetween the current 2'. in the diode 16 and the voltage V, across it. Solong as the diode is operating to the right of the ordinate i. therewill be no rectification but if the voltage swing across the diode isgreater than that shown in curve 20 (assuming that the voltage swingtakes place about the centre line V the diode will rectify and thusprevent further decrease in the minimum potential difference across thedischarge tube 5, this potential difierence being slightly above thecut-off voltage of the tube 5.

The resistance 15 assists in preserving a linear relation between V, andi. in the diode 16 and so reduces distortion.

I claim:

1. In an electrical apparatus, the combination of an electric dischargelamp having a predetermined minimum operating voltage, a source ofelectrical oscillations of varying voltage amplitude, means for applyingsaid electrical oscillations to said lamp, a source of constantoperating voltage for said lamp, said last named voltage being-at leastequal to the maximum desired voltage amplitude of the applied electricaloscillations plus said minlmum operating voltage of the lamp, and meansinterposed in circuit between said source of electrical oscillations andsaid lamp for limiting the voltage amplitude of said oscillationsapplied to the lamp to a value not grgater than said maximum desiredampli- 2. In an electrical apparatus, the combination of an electricdischarge lamp having a predetermined minimum operating voltage, asource of electrical oscillations of varying voltage amplitude, meansfor applying said electrical oscillations to said lamp, a source ofconstant operating voltage for said lamp, said last named voltage beingat least equal to the maximum desired voltage amplitude of the appliedelectrical oscillations plus said minimum operating voltage of the lamp,means interposed in circuit between said source of electricaloscillations and said lamp for limiting the voltage amplitude of saidoscillations applied to the lamp to a value below a predetermined valuenot exceeding the difference between the applied constant operatingvoltage and the mlnimum operating voltage, and means for adjusting saidlast named limiting means to respond to sald predetermined value.

3. In an electrical apparatus, the combination of an electric dischargelamp having a predetermined minimum operatin voltage, a source ofelectrical oscillations o varying voltage amplitude, means for applyingsaid electrical oscillations to said lamp, a source of constantoperating voltage for said lamp, said last named voltage being at leastequal to the maximum desired voltage am litude of the applied electricaloscillations p us said minimum operating voltage of the lamp, and meansincluding an electric valve interposedin circuit between said source ofelectrical oscillations and said lamp for limiting the voltage amplitudeof said oscillations applied to the lamp, said 'means being adjustablewhereby the anode current of said valve is reduced on each negative halfwave substantially to zero in response to oscillations of said maximumdesired amplitude.

4. In an electrical apparatus, the combination of an electric dischargelamp having a predetermined minimum operating voltage, a source ofelectrical oscillations of varyin voltage amplitude, means for applyingsai electrical oscillations to said lamp, a source of constant operatingvoltage for said lamp, said last named voltage being at least equal tothe maximum desired voltage amplitude of the applied electricaloscillations plus said minimum operating voltage of the lamp, and meansinterposed in circuit between said source of electrical oscillations andsaid lamp for limiting the voltage amplitude of said oscillationsapplied to the lamp, said means including a rectifier in series withsaid 1am and regulating means in circuit with said rectifier forpreventing its operation in response to electrical oscillations below apredetermined permissible high amplitude.

5. In an electrical apparatus, the combination of an electric dischargelamp having a predetermined minimum operatin voltage, a source ofelectrical oscillations o varying voltage amplitude, means for applyingsaid electrical oscillations to said lamp, 0. source of constantoperating voltage for said lamp, said last named voltage being at leastequal to the maximum desired voltage amplitude of the applied electricaloscillations, plus said minimum operating voltage of the lamp, astabilizing impedance in circuit with said lamp and source of operatingvoltage, and ad ustable means including a biased electric valve interosed in circuit between said source of electrical oscillations and saidlamp for limiting the voltage amplitude of said oscillations applied tothe lamp to a value not grgater than said maximum desired amplitu e.

In testimony whereof I have signed my name to this specification.

CECIL OSWALD BROWNE.

